Effluent treatment

ABSTRACT

A method of treating aqueous effluent containing caustic soda and oxychlorine compounds comprising filling the anode and cathode compartments of an electrolytic diaphragm cell with effluent to provide an electrolyte, electrolysing the effluent and reducing the oxychlorine compounds to chloride ions and hydroxyl ions at the cathode, recovering chlorine from the anode compartment and sodium hydroxide solution from the cathode compartment while allowing the effluent to flow through the cell by introducing effluent into the cathode compartment.

United States Patent t Stander Apr. 1, 1975 {54] EFFLUENT TREATMENT 2H1670 1/1939 Eichelberger 204/153 2 27( .3 942 204 3 1 1 2401.412 to,team 6. 20411.23

Kempton Park. South Attic-ti [73] Assignee: AE & CI Limited.Johannesburg. Primary Examiner-G. L. Kuplan South Africa AssistantErmniner-Auron Weisstuch Attorney, Agent or FirmCushman. Darby &

[22] Filed. June 7. i973 Cushman I21] Appl. No.: 368.046

[57] ABSTRACT [30] Foreign Application Priority Data A method oftreating aqueous effluent containing June :6 i972 South Africa 72 4407Caustic Soda and Oxychlvrine Compounds p gv filling the anode andcathode compartments of an 5 US. Cl H 204/98, 204/99. 204/128electrolytic diaphragm cell with effluent to provide an 2 4 53electrolyte. electrolysing the effluent and reducing the 5 1 1 Int CL II um Culh 7 o 1/40 oxychlorine compounds to chloride ions and hydroxyl[58] n w of Search H 304/98 99 g 5 ions at the cathode. recoveringchlorine from the 2 7 1 4 o 97 anode compartment and sodium hydroxidesolution from the cathode compartment while allowing the cf- 5References Cited fluent to flow through the cell by introducing effluentUNITED STATES PATENTS into the cathode compartment. morass 6/1936Ovcrdick ct ul. 204/153 7 Claims. 2 Drawing Figures l'his inrcntionrclatcs to thc trcatmcnt in clcctrolysis ccll of cfflucnts containingcaustic soda and o\ chlorinc compounds.

Aqucous cfflucnts containing caustic soda and oxychlorinc compounds suchas chloritcs. hypochloritcs and chloratcs arc produccd in man industrialproccsscs. Onc csamplc of such proccsscs is thc manufacturc ofchlorocarlmns.

Thcsc cfl'lucnts also contain considcrahlc uantitics of chloridc.

'l'hc chlorinc and caustic soda containcd in thc cfflucnts arc productswhich can hc rccyclcd into thc proccss from which thc compoundsoriginatc. if thc can hc rcco\crcd h cconomical mcthods.

('hlorinc and caustic soda arc normally prcparcd h clcctrofvsing a hrincsolution in clcctrohsis cclls.

Two typcs of cclls arc uscd: thc flowing mcrcury cathodc ccll and thcdiaphragm ccll.

ln thc mcrcury ccll. chlorinc is formcd at thc anodc and sodium amalgamat thc cathodc. ('austic soda is thcn ohtaincd h thc dcconiposition ofthc sodium amalganr Howcwr. thc mcrcur cathodc ccll \\ill not hcsuitahlc for thc rcco\ cr ot'caustic soda and chlorinc from cfl'lucntscontaining oxychlorinc compounds. sincc thc hypochloritcs and chloritcs\\ill hc oxidiscd to chloratcs and thc cfflucnt prohlcm will not hcsolrcd.

ln thc diaphragm ccll thc caustic soda is formcd in thc cathodccompartmcnt of thc ccll and chlorinc in thc anodc compartmcnt. 'lhccathodc and anodc com partmcnts arc scparatcd h a diaphragm to avoid thcformation of hypochloritc and chloratc in thc anodc compartmcnt h thcrcaction hctwccn chlorinc and h droxyl ions. Hydrogcn is also formcd atthc cathodc and thc diaphragm also scrrcs to scparatc thc hydrogcn andchlorinc formcd in thc ccll. 'lhc hrinc is fcd to thc anodc compartmcntto countcract thc migration ofthc hydrosyl ions from thc cathodccompartmcnt to thc anodc compartmcnt.

l'o rccorcr chlorinc and caustic soda from thc cfl'lucnts rcfcrrcd toahmc. this mcthod of clcctrolysis cannot hc uscd sincc h pochloritc andchloratc arc prcscnt thcrcin and clcctrolysing thc cfflucnt in adiaphragm ccll as uscd in hrinc clcctrolysis will favour thc futmtb tionof chloratcs and crcn pcrchloratcs in thc anodc compartmcnt.

It is an ohjcct of thc prcscnt inrcntion clcctrolytically to dccomposccfl'lucnt containing caustic soda and oxychlorinc compounds and torccovcr caustic soda and chlorinc from such cfflucnt. which products itdcsircd. ma hc rccyclcd to thc proccss from which thc cfflucntoriginatcs.

According to thc prcscnt in\'cntion a mcthod is prt ridcd of trcatingaqucous clflucnt containing caustic soda and o\ \chlorinc compoundscomprising filling thc anodc and cathodc compartmcnts of an clcctrolyticdiaphragm ccll \\ith cfllucnt to proridc an clcctrolytc. clcctl'oltsingthc cfflucnt and rcducing thc oxychlorinc compounds to chloridc ions andlt \Llt'ti\}l ions at thc cathodc. rccmcring chlorinc from thc anodccompartincnt and sodium Iodroxidc solution from thc cathodc compartmcutuhilc allowing thc cfflucnt to flow through thc ccll h introducingcftlucnt into thc cath odc compartmcnt.

Ill

'lhc anodc conipartmcnt ofthc ccll may hc fillcd ith an acidificd hrincsolution instcad of cfflucnt. to a\oid thc formation ofoxygcn at thcanodc in thc initial stagc of thc clcctrolysis causcd h thc prcscncc ofhydrosyl ions in thc anodc compartmcnt.

Prcfcrahly. thc cft'lucnt in thc anodc compartmcnt is acidificd withhydrochloric acid hcforc thc start of thc clcctrolysis to dccrcasc thclndroxyl ion conccntration and thus avoid thc initial formation ofo\ gcnat thc anodc.

Thc osychlorinc compounds arc prcfcrahh rcduccd at a cathodc madc ofiron or stccl. although othcr suitahlc cathodcs ma hc uscd.

Thc chlorinc ma hc lormcd at a carhon or graphitc anodc hilt it isprcfcrrcd to usc anodcs madc of titanium coalcd with a nictal sclcctcdfrom thc following clcmcnts. platinum. palladium. ruthcnium. rhodium andiridium.

'l'hc anodc and cathodc compartmcnts may hc scparatcd h a diaphragm madcof any matcrial suit-ahlc for immcrsion in a coi'rosirc liquid such as.for csamplc. \\'U\ cn polyvinyl chloridc cloth.

ln practising thc mcthod of this inrcntion. thc chlo rinc containingions prcscnt in thc cfflucnt will hc rcduccd to chloridc ions at thccathodc hcforc hydrogcn formation takcs placc. sincc thc oxidationpotcntials of thcsc ions arc highcr than that of thc hydrogcn ions. Thcchloridc ions \\ill thcn migratc to thc anodc compartmcnt whcrc chlorincgas will hc formcd at thc anodc. Hydroxyl ions will also tcnd to migratcto thc anodc. Howcvcr. thc hydrogcn ion conccntration in thc anodccompartnicnt is kcpt at a pH of about 4 in thc anodc compartmcnt and thchydroxyl ion conccntration will hc ncgligihlc.

Thc chlorinc is rcmovcd from thc anodc compartmcnt and this causcs aconccntration gradicnt of chloridc ions hctwccn thc cathodc and anodccompartmcnts. 'lhcrcforc. thc movcmcnt ofthc chloridc ions to thc anodccompartmcnt will hc fast and suhstantiall purc chlorinc gas will hcohtaincd from thc anodc compartmcnt. Hydroxyl ions and sodium ions whicharc prcscnt in thc cftlucnt will not hc rcduccd at thc cathodc and canhc rcniovcd from thc cathodc compartmcnt as sodium hydrosidc solution.'l'hc cft'lucnt is fcd continuously into thc cathodc compartmcnt.Hydrogcn formation at thc cathodc can hc aroidcd but. to lowcr thcchloridc contcnt of thc cfllucnt hy clcctrolysing thc sodium chloridcprcscnt thcrcin. hydrogcn is normally formcd at thc cathodc. (hlorinc isohtaincd from thc anodc compartmcnt and sodium hydroxidc solutionsuhstantialhv frcc of oxychlorinc compounds is ohtaincd from thc cathodccompartmcnt.

'lhc im'cntion will now hc dcscrihcd with rcfcrcncc to thc accompanyingdrawings. without rcstricting thc scopc of thc invcntion to thcparticular cmhodimcnts shown thcrcin.

FIG. 1 is a diagrammatic rcprcscntation of a laboratory typc diaphragmccll which was madc for usc in thc mcthod of thc prcscnt invention.

FIG. 2 is a scction through a lurthcr cmhodimcnt of a ccll according tothc invcntion.

In Fl(iv l. sidcs II and bottom 12 of thc ccll l0 wcrc constructcdofplatc glass. Thc lid l3 was madc of polytctraihiorocthylcnc whcrcinholcs wcrc providcd for thc conncctions to a mild stccl cathodc l4 and agraphitc anodc l5 thcrmomctcrs 16 for mcasuring thc tcmpcraturcs in thcanodc and cathodc compartmcnts l7 and 18. an inlet I) for the effluent.an outlet for the chlorine combined with an inlet 21 to introduce brine.if required. into the anode compartment 17. Further holes were providedin lid 13 for inlet 22 to introduce acid into the anode compartment 17.a pH-probe 23 to measure the pH of the anolyte and an outlet 24 forhydrogen. in one side I l of the cathode compartment 18. outlet 26 wasprovided to remove sodium hydroxide solution. The diaphragm 25 was apiece of woven polyv inyl chloride cloth mounted on a mild steel frame.

ln an experiment using the cell as shown in FIG. 1. the anodecompartment 17 was filled with a 5 percent by weight sodium chloridesolution the pH of which was adjusted to pH-& with hydrochloric acid. Anaqueous effluent containing sodium hydroxide. sodium hypochlorite andsodium chlorate was allowed to flow through the cathode compartment IS.The rate of flow and the current were so adjusted that hydrogen evolution just started at the cathode. indicating that all reducibleosychlorine ions had been reduced and that the reduction of water andthe formation of hydroxyl ions was proceeding.

The experiment was run during two consecutive days. hereby samples werecollected at the outlet 26 and were analysed iodomctrically forhypochloritc and chlorate ions and acidimctrically for hydroxide. Theresults of the two runs are given below:

The results show that a product free of oxychlorine I ions wherein thehydroxide content is increased can be produced from the effluent. whilechlorine is also produced.

The hydroxide solution and the chlorine are both suitable for re-use.

A further cell for use in the method of the invention was constructedout of dense concrete. The external dimensions ofthe cell were lot) mmwide. 440 mm long and 200 mm high.

Fl(i. 2 illustrates the position ofthe electrodes in this cell.

The cell 30 consists of cell body 31 and lid 32 con structed of 30 mmthick concrete coated with coal tar pitch.

The end walls and the bottom of cell body 3] worepro\ided with grooves33 to accommodate two cathodes 34 covered on one side with a wovenpolyvinyl chloride diaphragm 35.

The cathodes 34 were made of mild steel screening. having a wirediameter of 2 mm and a spacing of St) mm. welded into a mild steelframe.

The cathodes 34 were fixed in the grooves 33 of cell body 3| with coaltar pitch.

in this way. the cell 30 was divided into three compartments. onecathode compartment 56 and two anode compartments 37.

The anodes 38 were suspended from lid 32 and they were made of platinumcoated titanium.

A seal 39 was provided between the lid 32 and cell body 3! as well ascathodes 34 to prevent leakage from cell 30 and mixing of the gasesformed in the cathode and anode compartments.

The cell 30 was further provided with outlets 40 to remove chlorine fromthe anode compartments 37 and outlet 4| to remove hydrogen from thecathode com partment 36.

The lugs 42 on lid 32 were provided to connect the anodes to a source ofelectricity.

Other inlets and outlets (not shown) are provided in the end wallsofcell body 31. such as inlets for effluent to the cathode compartment36 and inlets for acid or anolyte to the anode com 'iartments 37.

Cathode compartment 36 is further provided with an outlet (not shown)for the caustic formed in the cell. Connections lnot shown) for theanodes 34 are also provided.

In an experiment using the cell 30 as illustrated in FIG 2. an effluenthaving the following weight/volume composition was fed to cathodecompartment 36.

Sodium hypochlotitc oi); li Sodium hydrmlde 3 v, Sodium chloride 13 Theanode compartments 37 were filled with the same effluent neutralized toa pH 4 with hydrochloric acid.

The effluent was clectrolyzed at a potential of 4.5 volt and a currentof ampere. which gave a current density of 3(a) A/m The rate of llow ofthe effluent was so adjusted that hydrogen evolution just started at thecathode. indicating that all reducible osychlorine ions had beenreduccd.

Samples taken from the flow from the cathode compartment 36 had thefollowing weight/volume composition:

Sodium hypoehlorii m Sodiuni hydroyide a l Sodium chloride 33 i; m/

Chlorine was evolved in the anode compartments. The results indicatethat this type ofeell i also suitable for use in the method of theinvention. The hypoehlorite was reduced efficiently and re-usablechlorine and caustic were produced.

The method of the invention provides an economically attractive methodfor the LlLLOl'nPUSlllUlt of aqueous effluents containing osychlorinecompounds whereby chlorine and caustic solution are produced suitablefor re-use.

I claim:

I. A method of treating aqueous effluent containing caustic soda andoxychlorine compounds comprising filling the anode and cathodecompartments of an electrolytic diaphragm cell with effluent to providean electrolyte. electrolysing the effluent and reducing the oxychlorinecompounds to chloride ions and hydroxyl ions at the cathode, recoveringchlorine from. the anode compartment and sodium hydroxide solution fromthe cathode compartment while allowing the effluent to flow through thecell by introducing effluent into the cathode compartment.

2. A method as claimed in claim 1 wherein the effluent in the anodecompartment is acidified.

6. A method as claimed in claim I wherein chlorine is formed at an anodemade of titanium cnated with a metal selected l'rum the followingelements. piaiinum. palladium. ruthenium. rhodium and iridiumv 7. Amethud as claimed in claim I wherein the cathnde and anude cumpartmemsare separated by a dia phragm made in woven pulyvinyl chloride cloth.

1. A METHOD OF TREATING AQUEOUS EFFLUENT CONTAINING CAUSTIC SODA ANDOXYCHLORINE COMPOUNDS COMPRISING FILLING THE ANODE AND CATHODECOMPARTMENTS OF AN ELECTROLYTIC DIAPHRAGM CELL WITH EFFLUENT TO PROVIDEAN ELECTROLYTE, ELECTROLYSING THE EFFLUENT AND REDUCING THE OXYCHLORINECOMPOUNDS TO CHLORIDE IONS AND HYDROXYL IONS AT THE CATHODE, RECOVERINGCHLORINE FROM THE ANODE COMPARTMENT AND SODIUM HYDROXIDE SOLUTION FROMTHE CATHODE COMPARTMENT WHILE ALLOWING THE EFFLUENT TO FLOW THRPUGH THECELL BY INTRODUCING EFFLUENT INTO THE CATHODE COMPARTMENT.
 2. A methodas claimed in claim 1 wherein the effluent in the anode compartment isacidified.
 3. A method as claimed in claim 2 wherein the solution oreffluent is acidified by the addition of hydrochloric acid.
 4. A methodas claimed in claim 1 wherein the oxychlorine compounds are reduced atan iron or steel cathode.
 5. A method as claimed in claim 1 whereinchlorine is formed at a carbon or graphite anode.
 6. A method as claimedin claim 1 wherein chlorine is formed at an anode made of titaniumcoated with a metal selected from the following elements, platinum,palladium, ruthenium, rhodium and iridium.
 7. A method as claimed inclaim 1 wherein the cathode and anode compartments are separated by adiaphragm made of woven polyvinyl chloride cloth.